In mobile communication networks according to the 3GPP (Third Generation Partnership Project) technical specifications, intermediate transport networks may be used to convey data between different nodes of the mobile communication network. One example of such a scenario occurs when using a technology referred to as High Speed Packet Access (HSPA). Here, a transport network, also referred to as lub transport network, may be used in the UMTS Terrestrial Radio Access Network (UTRAN, UMTS: Universal Mobile Telecommunications System) to couple a Radio Network Controller (RNC) to a radio access node (also referred to as Radio Base Station or NodeB). HSPA in the direction from the mobile communication network to a user equipment (UE) is also referred to as High Speed Downlink Packet Access (HSDPA), and HSPA in the direction from the UE to the mobile communication network is also referred to as High Speed Uplink Packet Access (HSUPA). The main aspects of HSPA are specified in the 25 series of the 3GPP technical specifications (e.g. the 3GPP technical specification 25.308).
According to HSPA, two levels of link layer retransmission protocols are used: Hybrid Automatic Repeat Request (HARQ) between the UE and the NodeB, and Radio Link Control (RLC) between the UE and the Radio Network Controller (RNC). A flow control protocol, also referred to as Framing Protocol (FP), has been introduced to control the sending rate of the RNC in the downlink direction and the sending rate of the NodeB in the uplink direction. The FP needs to address congestions on the radio link between the NodeB the UE and congestions in the transport network between the NodeB and the RNC, and will try to set an optimal sending rate which is as high as possible while avoiding the above types of congestion. This means that the FP attempts to control the sending rate in such a way that the fill level of queues in subsequent nodes will not exceed a given size.
However, it has turned out that the above-mentioned optimal sending rate is typically hard to achieve using the known FP. This is in part due to the known FP being a rate-based flow control protocol. Further, typical scenarios of using HSPA involve, e.g., a rapidly changing radio capacity available to a certain RLC connection or different lub transport network deployments with different RLC round trip delays. This may in result either in the sending rate being set too low or in congestions not being avoided. Both adversely affects end-to-end performance, e.g. measured in terms of throughput. Moreover, the known FP handles all RLC connections in the same manner. This may result in the end-to-end performance experienced by a high-priority service or user suffering from congestions in the lub transport network which are caused by RLC connections related to another service or user. The FP in HSDPA may also be configured to target fair bandwidth sharing among users or the target bit rate may be coupled to a user class so that some users get e.g. twice as much throughput as other users. In order to achieve this behavior, the HSDPA FP entity in the NodeB scales the target bit rates for each flow or user so that they match the desired relative bit rates. However, this still does not solve the above-mentioned problems of adapting the sending rate.
Another example of such a scenario occurs when using a technology referred to as Long Term Evolution (LTE). Here the transport network spans from the LTE base station, also referred to as E-UTRAN NodeB (eNodeB) in the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) to the Serving Gateway (S-GW) which is part of the Evolved Packet Core (EPC). The main aspects of LTE are specified in the 36 series of the 3GPP technical specifications (e.g. in the 3GPP technical specification 36.300).
Unlike HSPA, LTE does not use a link layer retransmission protocol between the eNodeB and the S-GW. Furthermore, no flow control protocol has been specified between these nodes. Instead, any node detecting excessive queuing may drop packets and thereby enforce the end-to-end congestion control mechanism of the transport layer as provided by the Transmission Control Protocol (TCP) to reduce its congestion window. This results in a reduction of the amount of data the TCP sender injects into the system and consequently, it reduces the size of the queue.
The principle of end-to-end congestion control by means of Queue Management is well known and widely used in the Internet. It cannot be applied in the HSPA transport network due to the above-mentioned link layer retransmission protocol which would recover from all packet drops and thereby hide the congestion from the TCP protocol.
While end-to-end queue management is, when applicable, superior to flow control protocols in terms of efficiency, performance and complexity, it does not provide per-user fairness or relative fairness. This is in particular the case when the bottleneck occurs in any transport network switch or router between the eNodeB and the S-GW as these are not aware of users or desired relative bit rate shares.
Accordingly, there is a need for techniques which allow for efficiently handling congestions in a transport network.